1. Field of the Invention
The present invention relates to optical phase modulators and, more particularly, to an optical phase modulator comprising a multi-layered dielectric stack configuration, with a terahertz signal bandwidth, capable of providing up to a full cycle of phase modulation in a nearly uniform manner for all contributing frequency components of transmitted optical signals.
2. Description of the Related Art
The use of optical signals for efficient power transmission and data conveyance across the extreme distances associated with the space environment is a highly desirable, yet elusive, capability. Advanced ultra-high-resolution applications, such as optical power beaming, interplanetary optical communication links, and extrasolar planetary imaging, could benefit from long-haul, free-space optical signal transport and detection. Common to these applications is the requirement for optical transmitters or receivers with large aperture baselines capable of maintaining wavefront coherency through active means.
As a quantification of the large aperture sizes required by these optical systems, reference is made to the case of NASA's Terrestrial Planet Finder (TPF) program. Typical TPF program concept descriptions specify a nullifying interferometric telescope configuration using free-flying collector elements to create a sparsely filled optical aperture baseline of up to one kilometer, providing a maximum angular resolution of 0.75 milliarcseconds at a wavelength of 3 μm. Imaging an extrasolar planet, rather than simply detecting it, requires even larger aperture baselines. As a comparative reference, the Hubble Space Telescope has a filled aperture of only 2.4 m in diameter. The next iteration, referred to as Next Generation Space Telescope, and currently slated for launch during the 2009 time frame, is planned to have a quasi-filled aperture diameter of 8 m.
The creation and deployment of such large optical apertures is extremely challenging. Not only must the aperture be of the appropriate diameter to satisfy resolution requirements, but it must also actively maintain wavefront coherency across the entire aperture area. For small optical systems with filled apertures, this requirement tends to be achievable through the serial alignment of conventional optical elements along and near the optical axis of the system. However, as the primary aperture and beam diameter increases, the ability to maintain required wavefront coherency across the filled lateral aperture area using traditional in-line optical elements becomes increasingly difficult.
The spatial segmentation of a large primary aperture into some optimal number of smaller aperture elements provides a useful approach to implementing large aperture systems. The segmentation strategy is particularly favorable for optical amplifier applications because it allows many smaller individual amplifiers to collectively create a large, scalable, effective aperture area of high average power. Aperture segmentation also eases the limiting factors associated with the space-based deployment and configuration of a single large filled aperture.
Segmentation does not, however, fully address issues associated with the maintenance of wavefront coherency and baseline configuration control across the aggregate lateral aperture area. Each segment in an aperture array must implement active correction of local wavefront phase variations in response to measurements taken from some full aperture reference field. By requiring each segment to modulate its local wavefront phase value to match a reference value, the array of individual segments may collectively create a single coherent wavefront of large beam diameter.
To be a viable approach for use in optical applications of transmissive geometry, the local wavefront correction technique implementation should have a rapid response, with a reaction time that is shorter than dynamic vibrational or thermal perturbations of the hosting structure. The correction technique implementation should also provide a relative transmitted phase modulation range of at least a full optical wavelength cycle, allowing for a modulo 2π correction of axial translations in position that may be any fraction of a wavelength in distance. For applications requiring precision transmission of pulsed power or high rate data, the correction technique implementation should supply a bandwidth wide enough to fully transmit all contributing frequency components of the primary optical signal with low loss and low distortion. Finally, as a major consideration in the design of space-borne systems, the overall mass of the correction technique implementation should be as low as possible, preferably much lower than the mass of state-of-the-art mechanical actuators commonly used to actively correct local figure variations in optical applications of reflective geometry.
Prior art devices used to implement wavefront correction or mirror figure control generally rely on mechanical actuations or nonmechanical devices such as liquid crystal spatial light modulators. It will be appreciated that mechanical actuators have the general disadvantage of being massive and relatively bulky. While other optically based phase modulation techniques may provide phase modulation for a particular frequency, these techniques implement a different level of phase modulation for other nearby frequencies, causing group velocity dispersion in pulses comprising a spectral spread of frequencies. Liquid crystal implementations have switching times that are on the order of milliseconds and are much slower than the bandpass modulation of the invention as described herein below.